When it comes to the use of military force, Americans tend to be in two camps: those who want to use overwhelming force to defeat our enemies and those who oppose the use of force for one reason or another.

Conservative hawks and Hollywood are in the first camp -- the bigger the bomb, the better. These folks would support President Franklin D. Roosevelt's call for unconditional surrender during World War II, even though the Vatican feared it would prolong the war.

In the second camp are those opposed to any killing (pacifists) and those who feel that no American should ever die helping a foreigner (isolationists). Both do not trust the government to use force well.

The limited use of force is denigrated by both sides. Foreign policy realists, on the other hand, see the use of military force as simply one among many tools of foreign policy. Thus, President John F. Kennedy could threaten the Russians during the Cuban missile crisis but at the same time secretly negotiate the withdrawal of our missiles from Turkey in exchange for removing the Russian missiles from Cuba.

Hawks criticized President George H.W. Bush, another realist, for not letting our troops take Baghdad. The hawks wanted total victory, and the pacifists opposed any fighting. Bush, on the other hand, had limited goals -- getting Iraq out of Kuwait. The limited use of force is a hard sell in a democracy. People don't want their children to die for limited foreign policy goals. Their deaths have meaning only if they are defending our country or dying for the highest principles (freedom, etc.).The French solved this problem by having the French Foreign Legion, an army made up of expendable foreigners. The U.S. tried to solve this problem by eliminating the draft and staffing the military with poor, undereducated minorities.

It is this context that makes it so difficult for Americans to understand the Vatican's position on the use of military force, which is based on the just war theory. The Vatican begins with a presumption against war. War can only be a last resort after all other avenues have been exhausted. Diplomacy and reconciliation must be tried first. But "last resort" does not mean "never."

Waging war requires a just cause, such as defending oneself or another from unjust aggression. But not every just cause is an excuse for the use of military might. Besides a just cause, the military intervention must cause less harm than not intervening. You do not destroy a village to save it. The use of military force must be proportionate, and everything possible should be done to avoid civilian casualties.

Popes John Paul II and Benedict XVI opposed both Gulf Wars and Pope Francis opposed any American intervention in Syria because they did not think those actions fulfilled the criteria demanded by the just war doctrine. They called for cease-fires, negotiations, diplomacy and reconciliation. They believed military intervention would only make matters worse.The popes were clearly right with regards to the second invasion of Iraq. It is hard to argue that the thousands of deaths and billions of dollars spent have made Iraq better.

As for Syria, former Secretary of State Hillary Clinton says she would have given more support to the "moderates" than President Barack Obama did, while the pope opposed any outside military intervention. I agree with the pope and Obama. There is no evidence that we would have done better in Syria than we did in Iraq, especially with a much smaller investment of resources. The moderates would have failed no matter how many weapons we gave them. The just war theory says you should not wage a war you cannot win.

Because the popes and the Vatican have so ardently opposed war, many were surprised when the Vatican supported intervention to stop the slaughter of religious minorities by the Islamic State. They should not have been surprised. The Vatican also supported international intervention in the early 1990s to stop ethnic cleansing in Bosnia-Herzegovina.Last week, Catholic News Service reported that when asked about the U.S. military airstrikes, Archbishop Giorgio Lingua, the Vatican nuncio to Iraq, told Vatican Radio, "This is something that had to be done, otherwise [the Islamic State] could not be stopped."

Likewise, Archbishop Silvano Tomasi, the Vatican representative to U.N. agencies in Geneva, said, "When all other means have been exhausted, to save human beings the international community must act. This can include disarming the aggressor." For Tomasi, this was a case of "humanitarian intervention," but it should be done by the international community and not unilaterally by one nation. The pope said something similar during his press conference on his way home from South Korea. In response to a journalist's question, he said:

In these cases where there is unjust aggression, I can only say that it is licit to stop the unjust aggressor. I underscore the verb "stop"; I don't say bomb, make war -- stop him. The means by which he may be stopped should be evaluated. To stop the unjust aggressor is licit, but we nevertheless need to remember how many times, using this excuse of stopping an unjust aggressor, the powerful nations have dominated other peoples, made a real war of conquest. A single nation cannot judge how to stop this, how to stop an unjust aggressor. After the Second World War, there arose the idea of the United Nations. That is where we should discuss: "Is there an unjust aggressor? It seems there is. How do we stop him?" But only that, nothing more.

Francis was very careful in what he said and what he did not say. In saying, "It is licit to stop the unjust aggressor," he said "stop," not "destroy," "conquer," "push back" or "defeat." Stop, "only that, nothing more." This limited goal will not please the hawks. Nor did he say how to stop the unjust aggressor, but he did say, "I don't say bomb, make war." Rather, "the means by which he may be stopped should be evaluated." And like Tomasi, he argues that this should be done by the international community, the United Nations.

Here, foreign policy realists will say that the pope is naive. The only way that the Islamic State was stopped was with force, including bombs. And if we had waited until the U.N. acted, it would have been too late to save anyone.For more on humanitarian intervention and the responsibility to protect, see the excellent analysis by my colleague Jesuit Fr. Drew Christiansen.

I would guess that the pope's diplomats (Lingua and Tomasi) are more closely articulating the Vatican's position than the pope. The pope is being extremely cautious because he does not want American hawks to say he is blessing American military intervention to destroy the Islamic State. Nor does he want Muslim extremists to say he is calling a crusade against Islam. He prudently errs on the side of caution and lets his diplomats fill in the blanks.

What the pope really did at his news conference was stress the word "stop." He is not giving the American military a blank check. Stopping the advance of the Islamic State allows for diplomacy and negotiations to take place. He understands that the use of American might to take back Mosul would be a disaster. Rockets and bombs can only liberate a city by destroying it.

From Obama's point of view, limited military intervention provides time for the new Iraqi government to get its act together, especially by bringing the Sunnis on board. He understands, as the hawks do not, that there is no American military solution to the conflict in Iraq. Only the Sunnis can defeat the Islamic State. After all, it was the Sunni awakening that defeated al-Qaida, not the American surge.

Here, the pope and the president agree: Only Iraqi negotiations and compromise can bring peace to the people of Iraq.